linux-kernel - Re: [PATCH] Tight check of pfn_valid on sparsemem

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Date:	Thu, 29 Jul 2010 13:55:19 -0700
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Christoph Lameter <cl@...ux-foundation.org>,
	Minchan Kim <minchan.kim@...il.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Milton Miller <miltonm@....com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>,
	Johannes Weiner <hannes@...xchg.org>,
	Kukjin Kim <kgene.kim@...sung.com>
Subject: Re: [PATCH] Tight check of pfn_valid on sparsemem - v4

On Thu, 2010-07-29 at 19:33 +0100, Russell King - ARM Linux wrote:
> And no, setting the sparse section size to 512kB doesn't work - memory is
> offset by 256MB already, so you need a sparsemem section array of 1024
> entries just to cover that - with the full 256MB populated, that's 512
> unused entries followed by 512 used entries.  That too is going to waste
> memory like nobodies business.

Sparsemem could use some work in the case where memory doesn't start at
0x0.  But, it doesn't seem like it would be _too_ oppressive to add.
It's literally just adding an offset to all of the places where a
physical address is stuck into the system.  It'll make a few of the
calculations longer, of course, but it should be manageable.

Could you give some full examples of how the memory is laid out on these
systems?  I'm having a bit of a hard time visualizing it.

As Christoph mentioned, SPARSEMEM_EXTREME might be viable here, too.

If you free up parts of the mem_map[] array, how does the buddy
allocator still work?  I thought we required at 'struct page's to be
contiguous and present for at least 2^MAX_ORDER-1 pages in one go.  

-- Dave

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